This invention refers to Al2O3 based layers having a total thickness of more than 0.3 mm produced on a substrate by plasma jet spraying.
Al2O3 layers produced by plasma jet spraying are used in the technology of electric insulation since years.
Thus, the publication DE 195 38 034 C1 describes a high temperature fuel cell comprising at least to ceramic layers of different composition arranged one upon the other, which layers can be produced by plasma jet spraying.
This sequence of different ceramic layers has the advantage that different requirements, such as electrical insulation and the coefficient of thermal linear expansion, are performed separately by different layers. As an example, ZrO2 and Al2O3 are mentioned as material for said ceramic layers.
Further, the publication U.S. Pat. No. 5,338,577 describes ceramic coated metallic substrates in which ZrO2 and Al2O3 are applied successively by plasma jet spraying.
Finally, the publication U.S. Pat. No. 4,588,655 describes a powder for plasma jet spraying consisting of Al2O3 and ZrO2.
Normally, a breakdown voltage of 15 kV per mm of layer is reached if no humidity is present. However, pure Al2O3 is a relatively brittle material which cannot be spread in layers having a thickness of more than 0.8 mm without incurring the risk of cracking.
It is a primary object of the present invention to produce Al2O3 based layers having a total thickness of more than 0.3 mm by plasma jet spraying, without generating macrofissures which would clearly reduce dielectric strength.
Another object of the present invention is to provide suitable alloy systems for producing said layers.
Still further objects of the invention will be evident from the following specification and claims.
The foregoing and other objects, advantages and features of the present invention can be attained by an Al2O3 based layer having a total thickness of more than 0.3 mm produced on a substrate by plasma jet spraying, said Al2O3 based layer having a laminar sandwiched structure wherein at least one Al2O3 layer is interpolated between two intermediate layers, which are produced by plasma jet spraying as well, said intermediate layers consisting of a ceramic laminated material which is different from Al2O3 and which on cooling increases in volume by phase transition.
This structure makes that the Al2O3 layers are compressed by the adjacent intermediate layers increased in volume, thus avoiding generation of macrofissures.
Preferred materials for said intermediate layers are Al2O3/ZrO2 and Al2O3/TiO2 alloy systems.
In a Al2O3/ZrO2 alloy system, on crystallization by phase transition, four crystalline phases are formed, i.e. xcex1-Al2O3, xcex3-Al2O3, cubic ZrO2 and monoclinic ZrO2, which together occupy an increased volume as compared with the original volume immediately after spraying. Thus, the intermediate layers pressurize the adjacent Al2O3 layer. This alloy system normally comprises from 5 to 50 percent by weight, and preferably from 10 to 30 percent by weight, of ZrO2.
Similar conditions prevail when using the other alloy systems mentioned above and below. Thus, in a Al2O3/TiO2 alloy system preferably the corresponding rutiles are formed. This alloy system normally comprises from 1 to 50 percent by weight, and preferably from 5 to 18 percent by weight, of TiO2.
Other alloy systems useful as intermediate layers in the present invention are e.g.:
ZrO2/Y2O3, preferably comprising from 8 to 22 percent by weight of Y2O3;
Y2O3/ZrO2, preferably from 5 to 15 percent by weight of ZrO2;
ZrO2/MgO, preferably comprising from 5 to 30 percent by weight of MgO;
ZrO2/CeO2, preferably comprising from 10 to 15 percent by weight of CeO2;
ZrO2/CaO, preferably comprising from 2 to 10 percent by weight of CaO.
Preferably, the thickness of said intermediate layers is from 0.1 to 1 mm and the thickness of said Al2O3 layers is from 0.05 to 0.3 mm.
Said intermediate layers of the present invention show a greater tenacity than the Al2O3 layers. This improves the toughness of the total layer. Moreover, they are electrically insulating, thus supporting an improved dielectric strength.
The layers of the present invention may be sealed on their surface with an organic or inorganic material.
Intermediate layers of the present invention consisting of the aforementioned alloy systems, and in particular of an Al2O3/ZrO2 aforementioned alloy system, are particularly useful for coating so-called corona rollers.